The utilization of ash-containing carbonaceous residues and removal of ash from the system has long been a problem in coal hydrogenation processes. For example, coal hydrogenation processes, in which coal is fed to the hydrogenation reactor in the absence of a pasting oil, produce gases, light oils, heavy oils, and tars. The light oils can be removed from the heavy oils and tars by vaporization or flesh evaporation and are thereby essentially freed of ash or other constituents such as tar or carbon. The remaining heavy oil-tar phase then contains the ash from the coal as well as unreacted coal or carbon particles.
In numerous hydrogenation processes attempts have been made to free the heavy oil-tar from solid matter by filtering or centrifuging. This has been found to be exceedingly difficult and very slow, requiring a great deal of equipment, resulting in a costly operation. In general these methods are regarded as highly unsatisfactory for the production of large volumes of liquid fuels from coal such as may be required in plants producing in excess of 50,000 barrels per day, as is presently contemplated.
One method for eliminating the heavy oil-solids separation step is to vaporize the light oil from the heavy oil in the hydrogenation process and then to continue the hydrogenation of the heavy oil-tar until it also becomes a distillable product. The light oil must be separated from the heavy oil and removed from the hydrogenation zone before further hydrogenation of the heavy oil-tar is undertaken, or much of the light oil will be hydrogenated to hydrocarbon gases. The velocity of the hydrogen-containing gases through the heavy oil hydrogenation zone will carry both the distillable oils which are formed, as well as ash and other solid, out of this zone. Since the ash and unreacted carbon are now in a gas phase, the solids may be removed from the gas in a cyclone separator. This method of separating the solids from oil vapors is shown in U.S. Pat. No. 3,926,775 (Dec. 16, 1975, by W. C. Schroeder).
It should be noted, however, that this method of solids removal cannot be used in hydrogenation processes in which the coal is fed to the processes as a slurry of oil and coal, since in essence it eliminates most or all of the heavy oil required in a slurry process. It is evident, therefore, that this method of ash removal is applicable only to hydrogenation processes in which coal is fed as a pulverized solid, as is the case in U.S. Pat. No. 3,926,775.
A catalytic process for the hydrogenation of coal has been proposed in which the residual materials are in the form of solid char containing the catalyst. This material is fed into a partial oxidation reactor where CO and H.sub.2 are generated and catalyst is said to be vaporized. The exit gases from the vaporizer are fed to the hydrogenator and ash is removed from the bottom of the reactor. U.S. Pat. No. 3,729,409 (Apr. 24, 1973 to Camp et al).